A typical ECA probe includes a plurality of coil elements arranged side-by-side on a base of usually printed circuit board (PCB) or other plastic material. Each coil element functions as an individual eddy current sensor which is coupled via a cable or the like to a data acquisition and processing module. In a typical inspection operation, an ECA probe is moved over a surface of a body being tested, e.g. the exterior surface of a pipe, or the fuselage of an aircraft. When the probe passes over a flaw, such as a crack or the like, each coil element passing over the flaw generates a signal, the characteristics of which can be used to determine the location and the size of the flaw.
A typical ECA probe often has two rows of coils that are staggered with each other to better detect defects with an orientation parallel to scan direction.
Ideally, to detect flaws with as high a resolution as possible using an ECA probe, each coil element should follow the contour of the surface of the body under non-destructive testing. In the past, rigid probes shaped to follow the contour of a surface, have been used to obtain a close matching of the probe and the surface being tested. Such a solution, however, has been limited to a few high volume applications because it requires the probe to be custom made for the surface of the body that is to be tested. This solution is not economical for low volume applications.
Thus, there has been a need for an ECA probe that is suitable of being used in testing for a body having a surface with a varying profile in all three dimensions (referred hereinafter as a 3D-shaped surface).
Existing efforts addressing the need of matching 3D-shaped test object surfaces include flexible ECA probes. Such prior art flexible ECA probes include a flexible base. The objective of these known probes is to obtain a probe capable of conforming to a 3D-shaped surface in order to obtain as high a resolution as possible.
One limitation of a prior art flexible ECA probe is its inability to bend around more than one direction simultaneously, which may impede an operator from accurately testing an object. This limitation is particularly severe when the bending direction is transverse to one another.
Another typical problem associated with another ECA probe relates to the staggering of the different rows coils. The staggering arrangement is often required to obtain 100% scanning coverage of the surface of the test object body because each coil is generally capable of detecting a defect that is within 50% of the coil's radius. This means that the center points of coils of one row are offset from the center points of the coils of the adjacent row. As a result, the natural bending line between two coils in one row of coils runs right through the center of a coil of the adjacent row of coils. That is to say, the natural bending lines between any two elements in one row of coils are stopped or impeded by the coils of the adjacent row. As a result, the probe is prevented from bending at natural bending lines roughly perpendicular to the rows of the elements.
Thus, given that existing flexible probes can not be bent at directions roughly perpendicular to the row of coils, the bending of the probe can only be attained through the deformation of the base of the probe, which may lead to the failure of the probe.
To overcome the problem noted above, ECA probes have been proposed in which the coils are embedded in a flexible circuit board or printed on a flexible circuit board. Examples of such proposals are disclosed in U.S. Pat. Nos. 7,078,895, 7,012,425, 6,954,065, 6,545,467, 6,452,384, 6,351,120, 6,288,537, 6,114,849, 5,841,277, 5,801,532, 5,389,876, and 5,047,719. The drawbacks of these printed or embedded coils are that the number of turns in each coil is limited by the PCB fabrication. Thus, it is not possible to build efficient low frequency coils. Moreover, it is not possible to integrate ferrites or coils with a winding direction not parallel to the surface of the probe.
Yet another prior art solution exists in which coils are glued to the surface of a substrate and each is individually connected to a data cable. A probe of such design includes the following drawbacks. First it can not be bent substantially since the wiring needs to move behind the coils. Secondly it is not possible to build a reliable high resolution probe because cables tend to break. Thirdly, it is not possible to apply pressure directly behind the coils as such pressure is exerted on the unstructured cable and hence reduces the quality of the signal. Lastly, this design presents the same problem of not being able to bend across the coil row direction.